It is a general problem that electronic devices and/or systems emit electromagnetic interference (“EMI”) over various spectral bandwidths and the EMI emissions can be disruptive to the operation of the electronic device or system itself, or to independent but locally operating electronic devices or systems. While the EMI may be generated purposefully for various reasons, EMI is naturally produced by various electronic operations such as by fast current variations or the like. These can occur during the normal operation of electrical components such as resistors, capacitors, inductors, transistors, diodes, or the like. Alternately, fast current variations can be generated during the normal operation of motive electrical devices such as motors, solenoids or the like. In addition to EMI being generated by electrical components and motive electrical devices, current fluctuations along conductive pathways, e.g., through wires, bus bars, conductive traces, or the like leading to and from electrical components and motive electrical devices can also generate disruptive EMI emissions.
Generally, EMI emissions produce disruptive signal noise. The disruptive signal noise is generally at the EMI emission frequency and at harmonics thereof. Radio frequency interference (“RFI”) bands, e.g., 50-1000 MHz, can be particularly disruptive to the operation of many devices by inducing disruptive radio signal noise into their otherwise noise-free signals. In particular, the operation of radio communication devices such as radio and television broadcasting systems, cellular telephone systems, and wireless computer network computers, and the like, can be adversely affected by RFI emissions emitted by unrelated devices.
Accordingly, EMI, and especially RFI emissions, in end-user products must be shielded or otherwise attenuated in a manner that reduces harmful EMI and RFI emissions to amplitude levels that are below the agreed upon EMI and RFI signal amplitudes at various signal bandwidths. Such amplitude levels are set by various regulatory agencies. Similarly, end-user products must also be capable of operating correctly when subjected to these same agreed upon EMI and RFI signal amplitudes levels at various bandwidths.
Additional problems are encountered when electronic devices and/or systems are used in harsh military or commercial environments. For instance, exposure to moisture, such as salt water or fog, may lead to corrosion or ingress of fluids. Accordingly, another desirable function of electronic device and/or system housings or packages is to resist corrosion and prevent fluid entry. Such systems that implement seals require a smooth interface to prevent water/moisture ingress. Most conductive paints are rough because of the metallic content of the paint.
Other functions of electronic devices and/or system housings or packages are to eliminate electrical shock hazards and to protect enclosed elements from damage by mechanical shock and or high g-forces. Other desirable functional characteristics of electronic device and/or system housings or packages are lightweight, easy of use, low cost, easy of manufacture and aesthetic appearance. Other desirable functional package characteristics may include forming a gas or liquid tight seal, ease of operator access, e.g., to replace a battery or the like, and a variety of convenient and functional interface access ports for providing electrical, mechanical, and other interfaces between the packaged electronic device or system and external devices or systems.
U.S. Pat. No. 6,763,576 to Watchko et al. discloses coating an interior surface of a housing with a metalized electrically conductive EMI shielding layer, which can include tin, nickel or an alloy of the two to resist corrosion. The housing can be made from magnesium or aluminum, among other metals, or plastics. The metal portion of the housing may be painted with a non-conductive paint. A conductive gasket may be used to provide electrical continuity between various mating housing parts. However, the reference does not disclose any means for protecting the exterior surface from corrosive materials, and the reference does not disclose masking any portion of the interior surface.
U.S. Pat. No. 7,446,265 to Krohto et al. discloses that electromagnetic interference shields may include an electrically conductive and grounded housing. The housing may be formed of a metal such as steel, aluminum, magnesium, or alternatively, a plastic or other polymeric material which is filled to form an electrically-conductive material. A conductive coating, such as an electrically-conductive paint, may be generally applied across the interior surfaces of the housing. A conductive gasket may be used to provide electrical continuity between different housing components. However, the reference does not disclose any means of protecting either the interior or exterior surface from corrosive materials, and the reference does not disclose masking any portion of the interior surface.
U.S. Patent Application Publication No. 2008/0273316 to Sarno et al. discloses a magnesium. Faraday shield, and U.S. Pat. No. 5,909,490 to Sokolich et al. discloses a magnesium EMI shield for a telephone. These references disclose the use of magnesium in EMI shields due to its low density and electrical conductivity. However, the references do not disclose any means of protecting either the interior or exterior surfaces from corrosive materials, and the references do not disclose masking any portion of the interior surface.
U.S. Pat. No. 4,988,550 discloses a masking tape for EMI shielding applications comprising, among other things, an adhesive layer and a metal foil layer adhered to the top of the adhesive layer. The tape is applied to a substrate that is sprayed with a paint, and the mask is then removed to reveal a conductive metal foil surface to which a conductive EMI shield can be mounted. However, the reference does not disclose a means of protecting both the interior and exterior surfaces from corrosive materials.
Thus, there still remains a need for an electronic package that comprises an electromagnetic shield, exhibits corrosion resistance on both the interior and exterior surfaces, and forms a water-tight seal.